The present invention relates generally to terminal equipment for an optical communication system. More specifically, the present invention relates to optical transmitters multiplexing and optical receivers demultiplexing signals having substantially equal dispersion.
U.S. Pat. No. 5,224,183, entitled xe2x80x9cMultiple Wavelength Division Multiplexing Signal Compensation System and Method Using Samexe2x80x9d and issued on Jun. 29, 1993, discloses a wavelength-division multiplexing (WDM) system. FIG. 1 illustrates a wavelength-division multiplexing system disclosed in U.S. Pat. No. 5,224,183. As FIG. 1 illustrates, each wavelength has an associated laser coupled to a dispersion-compensation fiber, which in turn is coupled to a common wavelength division multiplexer. For example, lasers 12, 14 and 16 are coupled to dispersion-compensation fibers 18, 20 and 22, respectively, which are coupled to wavelength division multiplexer 24. In this example, the wavelength of laser 12 is 1540 nm; the wavelength of laser 14 is 1550 nm; the wavelength of laser 16 is 1560 nm. Wavelength division multiplexer 24 is coupled to a an additional dispersion-compensating fiber 26 and transmission fiber 28.
This known system individually compensates the dispersion associated with each wavelength before the optical signals are multiplexed by the wavelength division multiplexer (and after the optical signals are demultiplexed by the wavelength division demultiplexer (not shown in FIG. 1)). This is performed for each wavelength by a separate and unique dispersion-compensation fiber associated with that wavelength: dispersion compensation fiber 12 has a dispersion of xe2x88x9220 ps/nm at its wavelength (1540 nm), dispersion-compensation fiber 14 has a dispersion of xe2x88x92200 ps/nm at its wavelength (1550 nm), and dispersion-compensation fiber 16 has a dispersion of xe2x88x92360 ps/nm at its wavelength (1550 nm). These dispersion-compensation fibers compensate individually for each particular wavelength to produce a unique residual dispersion associated with each wavelength. Each wavelength is subsequently compensated by the dispersion-compensation fiber 26 and transmission fiber 28. By eliminating the residual dispersion associated with each wavelength at the wavelength-division multiplexer 24, the dispersion of all of the wavelengths at the end of the transmission fiber 28 can be controlled to a desired amount, such as for example, approximately zero dispersion for approximately all of the wavelengths.
Such a WDM system, however, suffers several shortcomings. First, each wavelength requires a separate and unique dispersion-compensating fiber disposed, for example, between the respective laser and the wavelength-division multiplexer of the optical transmitter. Similarly, each wavelength requires a separate and unique dispersion-compensation fiber disposed, for example, between the wavelength-division multiplexer and the respective detector (not shown in FIG. 1). As WDM systems having more and more information channels are designed, adding more and more dispersion-compensation fibers associated with each wavelength make the WDM system more complex and expensive.
Second, polarization of the optical signals received by the wavelength-division multiplexer cannot be maintained due to the unique dispersion-compensation fibers required by each wavelength. Consequently, although desirable for the optical signals associated with each wavelength to have an associated polarization that is orthogonal to adjacent wavelengths, such an arrangement is not possible where the polarization cannot be maintained.
An apparatus for communicating data through information channels each being associated with its own wavelength comprises modulators and an optical multiplexer. Each modulator is associated with its own wavelength. The optical multiplexer is operationally coupled to the modulators. The optical multiplexer receives multiple input optical signals each of which is received from its own modulator. Each input optical signal has its own dispersion substantially equal to a dispersion of each remaining input optical signals.